The invention relates to axial flow fans and more particularly to the reduction of axial fan blade passing frequency tone in engine cooling applications.
The sound quality of axial fans is strongly influenced by the blade passing frequency (BPF) tone and harmonics generated by the blades. This BPF can be calculated from the blade passing order. The blade passing order corresponds to the number of blades multiplied by the speed of rotation in RPM divided by 60. Conventionally, these BPF tones can be minimized by skewing the blades (forward or backward), by using leaned blades and by using unequal blade spacing. However, the amount of skewing and leaning is always limited by the fan structural integrity and the unequal spacing is limited by the space available between the blades at the hub. As a consequence, the BPF tone and harmonics are still a major concern in today""s fan sound quality.
Accordingly, there is a need to provide an axial flow fan that reduces the axial fan blade passing frequency (BPF) tone and harmonics for use in any engine cooling application requiring at least one axial fan.
An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is achieved by providing a low tone axial flow fan structure including a hub; an inner fan including a number of inner blades attached to the hub and extending radially outwardly to a first circumferentially extending blade support structure; and an outer fan including a number of outer blades attached to the first circumferentially extending blade support structure and extending radially outwardly to a second circumferentially extending blade support structure. The number of outer blades defines an outer fan blade passing order. The number of inner blades ensures creation of blade passing orders or multiple of said blade passing orders within one blade passing order of the outer fan blade passing order so as to provide masking of the outer fan blade passing order.
In accordance with another aspect of the invention, a low tone axial flow fan structure includes a hub; an inner fan including a number of inner blades attached to the hub and extending radially outwardly to a first circumferentially extending blade support structure; and an outer fan including outer blades. The number of outer blades is the same as the number of inner blades, with each outer blade being generally adjacent to an inner blade. The outer blades being attached to the first circumferentially extending blade support structure and extending radially outwardly to a second circumferentially extending blade support structure. A leading edge of each inner blade is shifted with respect to a leading edge of an adjacent outer blade.
Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon-consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.